import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution3 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return bool布尔型
     */
    public boolean isValidBST (TreeNode root) {
        // write code here
        ArrayList<Integer> list = new ArrayList<>();
        preorder(root,list);
        for(int i = 0;i<list.size()-1;i++) {
            if(list.get(i) > list.get(i+1)) {
                return false;
            }
        }
        return true;
    }
    public void preorder(TreeNode root,ArrayList<Integer> list) {
        if(root == null) {
            return;
        }
        preorder(root.left,list);
        list.add(root.val);
        preorder(root.right,list);
    }
}